Particles higher than geometry
The Simple way ... the Origin

The placement of the particle uses the origin of the particle.  In edit mode move the mesh vertices away from the origin.  You can easily experiment which axis works best for your situation. 

X or Y or Z 

The origin of the mesh need not lie anywhere near the center of mass of the mesh.

For example a sphere particle may have its origin twice the distance of the diameter ... or in the geometric center.   A tree form particle probably would have its origin at the base of the trunk ... not in the geometric center.